The present application claims priority to Chinese application having serial number 01113750.9 entitled xe2x80x9cNovel method of temperature compensation for interleaverxe2x80x9d, filed on Jul. 2, 2001 which is incorporated herein by reference
The present invention relates generally to the configuration of optical retarders, and in particular to methods for configuring temperature insensitive optical retarders using birefringent crystals.
Optical retarders (waveplates) is one of the most common optical elements with wide applications in solar system observation such as birefringent filters, display, and telecommunication systems. Especially, with the recent rapid progress in optical telecommunication systems, optical retarder has become one of the important devices for building advanced optical components. One of the applications of the optical retarders in the optical communication systems is in construction of optical interleavers.
As telecommunications usage increases as a result of, for example, increased Internet usage, increased types of communications, population growth, etc., telecommunications providers are required to provide greater voice- and data-carrying capacity. In order to reduce cost and the amount of time required to provide the increased capacity, wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have been developed, which provide increased capacity without requiring new fiber optic cables. WDM and DWDM technologies combines multiple optical signals into a single fiber by transporting each signal on a different optical wavelength or channel. Multiplexing and demultiplexing of optical channels is typically accomplished with thin film optical filters. However, multiple layers of film are required to multiplex and demultiplex multiple channels, which increases the cost and complexity of a component.
One solution is the use of interleaver technology, and in particular to the birefringent waveplate-based interleaver technology disclosed in U.S. Pat. No. 4,566,761 issued Jan. 28, 1986; and U.S. Pat. No. 4,685,773 issued Aug. 11, 1987 both to Carlsen et al. Optical interleavers multiplex and demultiplex a plurality of optical channels into odd and even channels with the spacing twice larger than that of the original channels.
One problem associated with the conventional birefringent waveplate (retarder) based interleaver is the temperature sensitivity due to the refractive index change of the birefringent crystal. To solve this problem, a temperature insensitive optical retarder (waveplate) that uses two different crystals with opposite temperature dependence of refractive indices is proposed. Although this solution works as intended, however, in practice, it is difficult to find an exact match between two birefringent crystals, and the compensation range is also limited due to nonlinearity of the refractive index change.
One of the objectives of the present invention is to provide a temperature insensitive optical retarder that is easy to construct and provides wide operating range.
Another objetive of the present invention is to provide a temperature insensitive interleaver.
In accordance with one aspect of the present invention, there is provided a temperature insensitive optical retarder using birefringent crystals comprising;
first and second wedge shaped birefringent crystals each having non parallel faces; and
a holding means for mounting said first and second birefringent crystals comprising a first material having a first thermal expansion coefficient and a second material having a second thermal expansion coefficient which is different from the first thermal expansion coefficient;
wherein the first and the second wedge shaped birefringent crystals are attached onto the first and second materials, respectively, so that the total retardance by the first and second birefringent crystals at a first temperature is substantially the same as that at a second different temperature at a given wavelength.
In accordance with another aspect of the present invention, there is provided a method for configuring a temperature insensitive optical retarder comprising the steps of;
providing a first wedge shaped birefringent crystal;
providing a second wedge shaped birefringent crystal;
providing a mount including a first material having a first thermal expansion coefficient and a second material having a second material;
mounting the first birefringent crystal to the first material and the second birefringent crystal to the second material, so that they are movable relative to each other;
wherein the first and second materials are selected so that the total retardance by the first and second crystal is substantially the same at a given wavelength over a given temperature range.
In accordance with another aspect of the present invention, there is provided an optical interleaver/deinterleaver for multiplexing two data streams into a stream of channels or for demultiplexing a stream of channels into two sets of data streams comprising;
at least first and second wedge shaped birefringent crystals each having non parallel faces; and
a holding means for mounting said first and second birefringent crystals comprising a first material having a first thermal expansion coefficient and a second material having a second thermal expansion coefficient which is different from the first thermal expansion coefficient;
wherein the first and the second wedge shaped birefringent crystals are attached onto the first and second materials, respectively, so that the total retardance by the first and second birefringent crystals at a first temperature is substantially the same as that at a second different temperature at a given wavelength.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention ill conjunction with the accompanying figures.